The field of the present invention concerns the treatment of a noisy time signal constituting an information support enabling characterisation of a set of events produced randomly by a source of events.
It is in fact known that a spectrum obtained from particle sources may comprise energy rays which are characteristic of it.
There may consequently be obtained from a measurement of the particle source, a spectrum whose examination by a specialist or by software enables information to be obtained about the said particle source.
Examination enables identification of the nature of the particle source.
By way of non-limiting example, in the field of gamma rays, a system of the said type may provide a ray spectrum such as shown in FIG. 1, which enables to identify the radioelements which compose this source and so to characterise the latter.
It will be noted as an indication that FIG. 1 shows in particular a normalised energy spectrum for cesium 137.
In order now to explain a typical operation of state of the art systems mentioned above, an example in the field of gamma spectrometry will be used.
One skilled in the art will of course be able to extend this example without difficulty to other categories of radiation such as those mentioned above.
FIG. 2 shows, by way of example, a signal which could ideally be observed immediately at the output of a gamma photon detector.
This signal comprises plural pulses of different amplitude and duration representing, for example, a current developed in the detector by the flow of a photon.
It will be noted here that a pulse may also represent a voltage in the detector.
In all cases, in the following text the signal provided by the detector will be termed the detector current signal.
To briefly return to the pulses, their width, corresponding to a certain time duration, is a function of the charge collection time.
As mentioned above, the detector current signal shown in FIG. 2 is ideal.
Consequently such a signal is never observable.
In reality, a preamplifier is generally installed at the output of the detector, in order to implement a first shaping of the detector current signal.
Two types of preamplifiers are generally found in the existing systems: preamplifiers with capacitive feedback and preamplifiers with resistive feedback.
By way of indication, FIGS. 3 and 4 show a detector 1 respectively followed by a preamplifier with capacitive feedback 2 and by a preamplifier with resistive feedback 3 which comprises a feedback loop, composed of a capacitor 5 and a resistor 6 in parallel, between an output and an input of an amplifier 4.
These preamplifiers are generally followed by a differentiator circuit 7 in the case of a capacitive feedback and by a pole-zero PZ correction circuit 8 in the case of a resistive feedback.
The two FIGS. 5 and 6 respectively show an example of an ideal time signal at the output of the two said types of preamplifiers upon input excitation by the same detector current signal, it being understood that electronic noise is not shown here.
Several steps follow that of preamplification; their order and their implementation may vary considerably.
A known and important step consists of shaping the pulses to extract information of energy type therefrom.
Such a step is generally termed an “energy” shaping step.
By way of non-limiting example, information may be extracted from a measurement of the amplitude or of the area of each pulse.
In fact, it is supposed that these quantities are generally proportional to energy.
The choice of the type of “energy” shaping has in recent years been the subject of considerable research in the field of the invention; this delicate step necessitates numerous compromises.
For example, in the case of extracting information from the pulse amplitude, a compromise desires the existence of an optimum between:
obtaining the information very precisely, that is, a high resolution,
the number of pulses per unit time present in the detector current signal, and
the fraction among them which it is desired to retain in the spectrum, for example because of a phenomenon, known per se, which is currently termed in the field, “piling up” of pulses.
For more details, the reader will be able to refer particularly to the concepts of the spectrum input rate or output count rate (OCR) and of detector output rate or input count rate (ICR).
In order to obtain an optimum solution to this compromise, articles [1], [2] have established that the use of a filter of trapezoidal shape could constitute an optimum solution in the absence of piling-up of pulses and for a certain nature of noise.
Other solutions based on such a solution have later been proposed to improve system performance.
For example, numerous possibilities are known for realising a filter of this type [1-8]: numerical, analogue, or mixed types, etc.
Solutions are also known having the aim of improving performance by another means: optimisation of said PZ correction [8,9,10,11], optimisation of said conventional operation consisting of correcting a baseline [12], rejection of piled-up pulses by correlation between the length and amplitude of a pulse which is not piled up [13], etc.
But here also, the step of energy shaping necessitates the use of the optimum trapezoidal filter.
Summing up, the proposed methods and systems comprise in all cases a trapezoidal filter, and more generally an energy shaping step, which is a disadvantage.
In fact, the performance of systems of this type, although they have been very serviceable, are nevertheless limited, since in any case this energy shaping step, and in particular the use of the said trapezoidal filter, degrades the detector current signal—at least the preamplified signal—particularly by a lengthening of the signals in time.
Consequently, a moment always arrives at which, with the pulse occurrence frequency increasing (because of an increase of events frequency, for example particle emissions), systems of this type begin to malfunction, for example producing spectra of low resolution, or deformed.
This is typically the case towards 100,000 to 200,000 counts per second (counting rate).
For more efficient systems, 300,000-400,000 counts per second may only just be expected.
Another disadvantage of the said systems is that their use does not remain very flexible.
In particular, it is not possible to adapt or modify the energy shaping step parameters during the analysis of the radiation source.
It is therefore necessary in the initial phase to know, or at least to have the best possible estimate of, the number of pulses per second, for regulating the system, and this particularly at the detection level.
And if the number of pulses is overestimated, a shorter convolution is chosen, which will degrade the resolution.
While if this number of pulses is underestimated, a too long convolution is chosen, which will lead to rejecting many pulses and distorting the spectrum.
By way of non-limiting example, systems which because of their situation cannot uniquely be adjusted when placed in service are generally adjusted to a worst case.
Adjusting to a worst case can particularly consist of adjusting the system as a function of the strongest measured pulse intensity.
But this intensity varies during the course of time, so that the system no longer operates with an optimum adjustment.